Climate Change: Evidence & Causes

Climate change is one of the most important and controversial topics in GCSE Geography. The AQA specification requires you to understand the evidence for climate change, distinguish between natural and human causes, and evaluate the relative importance of each. This lesson provides the knowledge you need for a thorough and balanced answer.

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What Is Climate Change?

Climate change refers to a significant and lasting change in the average weather patterns over an extended period (typically decades to millions of years). The Earth's climate has always changed naturally, but the current period of warming is occurring at an unprecedented rate.

Key Definitions

Term	Definition
Climate	The average weather conditions in a region over a long period (30+ years)
Weather	The day-to-day conditions of the atmosphere (temperature, rainfall, wind)
Global warming	The recent rise in average global temperatures, particularly since ~1850
Greenhouse effect	The natural process by which certain gases trap heat in the atmosphere
Enhanced greenhouse effect	The intensification of the greenhouse effect due to human activities

Exam Tip: "Climate change" and "global warming" are not the same thing. Global warming refers specifically to rising temperatures; climate change encompasses all changes to climate patterns, including rainfall, wind, and extreme weather. Use the correct term in your answers.

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Evidence for Climate Change

Evidence comes from multiple independent sources, all pointing to the same conclusion: the Earth is warming rapidly.

1. Temperature Records

• Instrumental records (from thermometers) go back to the mid-1800s.
• Global average temperature has risen by approximately 1.1 °C since pre-industrial times (1850–1900).
• The ten warmest years on record have all occurred since 2010.
• The rate of warming is accelerating: the last 50 years have warmed at almost twice the rate of the previous 100 years.

2. Ice Cores

• Scientists drill deep into ice sheets in Antarctica and Greenland to extract ice cores.
• Trapped air bubbles in the ice contain samples of the atmosphere from up to 800,000 years ago.
• Analysis of these bubbles shows:
  • CO2 levels have fluctuated naturally between about 180 ppm and 280 ppm over glacial cycles.
  • Current CO2 levels (over 420 ppm as of 2024) are far higher than at any point in the past 800,000 years.
  • Higher CO2 levels correlate closely with higher temperatures.

3. Retreating Glaciers

• Mountain glaciers worldwide are retreating (shrinking) at an accelerating rate.
• Examples:
  • Glacier National Park, USA — had 150 glaciers in 1850; only 25 remained by 2015.
  • Kilimanjaro, Tanzania — has lost approximately 85% of its ice cap since 1912.
  • Alps — glaciers have lost about half their volume since 1900.
• Photographs taken decades apart provide striking visual evidence of glacial retreat.

4. Sea-Level Rise

• Global sea level has risen by approximately 20 cm since 1900.
• The rate of rise is accelerating: currently about 3.6 mm per year.
• Caused by two factors:
  • Thermal expansion — warmer water takes up more space.
  • Melting ice — glaciers, ice caps, and ice sheets (especially Greenland and West Antarctica) are losing mass.

5. Arctic Sea Ice

• The extent of Arctic sea ice in summer has declined by approximately 40% since satellite records began in 1979.
• The thickness of Arctic sea ice has also decreased significantly.
• Some scientists predict the Arctic could experience ice-free summers within the coming decades.

6. Other Evidence

• Tree rings (dendrochronology) — wider rings indicate warmer growing seasons; patterns going back thousands of years show temperature trends.
• Pollen analysis — pollen preserved in sediments shows which plants grew in a region, indicating past climate conditions.
• Sea-floor sediments — fossilised shells and organisms reveal past ocean temperatures.
• Historical records — paintings, diaries, harvest dates, and records of river freezing provide evidence of past climate conditions (e.g. the "Little Ice Age" of the 14th–19th centuries, when the Thames regularly froze).

Exam Tip: Learn at least three different types of evidence for climate change. The examiner wants to see that you understand evidence comes from multiple sources, not just temperature records.

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Natural Causes of Climate Change

The Earth's climate has changed many times throughout its 4.5-billion-year history due to natural factors:

1. Orbital Changes (Milankovitch Cycles)

The Earth's orbit around the Sun varies in three ways over long time periods:

Cycle	Period	Description
Eccentricity	~100,000 years	The shape of Earth's orbit changes from more circular to more elliptical
Axial tilt	~41,000 years	The tilt of Earth's axis varies between 22.1° and 24.5°
Precession	~26,000 years	Earth's axis wobbles like a spinning top

These cycles affect the amount and distribution of solar energy reaching the Earth and are the primary cause of ice ages and interglacial periods.

2. Solar Output

• The Sun's energy output varies slightly over an 11-year solar cycle (sunspot cycle).
• Periods of increased sunspot activity correspond with slightly more solar energy reaching Earth.
• However, changes in solar output are too small to explain the rapid warming observed since the mid-20th century.

3. Volcanic Activity

• Major volcanic eruptions inject ash and sulphur dioxide into the stratosphere.
• These particles reflect sunlight, causing short-term cooling (typically 1–3 years).
• Example: The eruption of Mount Pinatubo (Philippines, 1991) caused global temperatures to drop by about 0.5 °C for 1–2 years.
• Volcanic activity causes temporary climate fluctuations, not long-term warming.

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Human Causes of Climate Change (The Enhanced Greenhouse Effect)

The Natural Greenhouse Effect

The greenhouse effect is a natural and essential process that keeps the Earth warm enough to support life:

1. Short-wave solar radiation from the Sun passes through the atmosphere and warms the Earth's surface.
2. The Earth re-emits this energy as long-wave infrared radiation.
3. Greenhouse gases (GHGs) in the atmosphere absorb some of this outgoing radiation and re-emit it back towards the Earth.
4. This keeps the Earth's average temperature at about 15 °C (without the greenhouse effect, it would be about -18 °C).

The Enhanced Greenhouse Effect

Human activities since the Industrial Revolution (c. 1750) have dramatically increased the concentration of greenhouse gases in the atmosphere:

Greenhouse Gas	Main Human Sources	Contribution to Enhanced Warming
Carbon dioxide (CO2)	Burning fossil fuels (coal, oil, gas), deforestation	~65% of human-caused warming
Methane (CH4)	Agriculture (rice paddies, cattle), landfill sites, fossil fuel extraction	~16% of human-caused warming
Nitrous oxide (N2O)	Agricultural fertilisers, burning fossil fuels, industrial processes	~6% of human-caused warming
F-gases	Refrigeration, air conditioning, aerosols, industrial processes	~2% of human-caused warming
Water vapour	Evaporation (amplified by warming)	Amplifying effect (feedback loop)

This diagram summarises the key causes and effects of climate change:

graph LR
    A[Human Causes] --> B[Burning fossil fuels]
    A --> C[Deforestation]
    A --> D[Agriculture and livestock]
    B --> E[Increased CO₂]
    C --> E
    D --> F[Increased methane]
    E --> G[Enhanced greenhouse effect]
    F --> G
    G --> H[Rising temperatures]
    H --> I[Sea-level rise]
    H --> J[Extreme weather]
    H --> K[Ecosystem disruption]

Key Human Activities Driving Climate Change